The background art is explained below by using a hybrid shovel as an example.
FIG. 7 shows the system configuration of the hybrid shovel.
The hybrid shovel is provided with an engine 1, a generator-motor 2 and a hydraulic pump 3 connected to the engine 1, a hydraulic actuator 5 driven by pressurized oil discharged from the hydraulic pump 3, a control valve 4 controlling the supply and discharge of the pressurized oil to and from the hydraulic actuator 5, and an electric storage device 6 charged by the electric power generated by the generator-motor 2. In such a hybrid shovel, the engine 1 is assisted by actuating the generator-motor 2 by the electric power charged into the electric storage device 6.
In addition to the generator-motor 2 and the electric storage device 6, the hybrid shovel is also provided with a slewing motor 7, a first inverter 8, and a second inverter 9 as equipment (referred to hereinbelow as “hybrid equipment”) electrically connected to the electric storage device 6. The inverters 8 and 9 control the operation of the generator-motor 2 and the slewing motor 7 and also the charging and discharging of the electric storage device 6.
In such a hybrid shovel, sufficient power cannot be obtained at a low temperature because the electric storage device performance (discharge performance) is degraded due to the decrease in capacity of the electric storage device 6. For this reason, in a low-temperature environment, such as during the winter, it is desirable that the electric storage device 6 be also heated (warmed up) to an appropriate temperature (for example, 10° C. to 40° C.), similarly to the engine 1.
A technique for increasing the temperature of the electric storage device by internal heating of the electric storage device has been suggested as warming-up means (see Patent Document 1). More specifically, with the technique described in Patent Document 1, the generator-motor is actuated and the electric storage device is forcibly charged and discharged when the temperature of the electric storage device becomes equal to or lower than a preset value.
Meanwhile, where the electric storage device 6 is continuously used in a high-temperature state, the deterioration of the electric storage device 6 is accelerated. Therefore, it is necessary to cool the electric storage device 6 so that the temperature of the electric storage device 6 does not exceed the appropriate temperature.
Further, most of the hybrid equipment units, including the slewing motor 7 and inverters 8 and 9, also generate heat during usage. Therefore, the hybrid equipment also needs cooling in order to maintain the constant performance thereof.
A cooling circuit described in Patent Document 2 is known as means for cooling the electric storage device and the units of the hybrid equipment that need cooling (referred to hereinbelow as “equipment that needs cooling”). More specifically, with the cooling circuit described in Patent Document 2, the electric storage device and the equipment that needs cooling can be cooled by supplying cooling liquid by a cooling pump to the electric storage device and the equipment that needs cooling.
A problem associated with the technique described in Patent Document 1 is that the service life of the electric storage device 6 is shortened because the electric storage device 6 is charged and discharged at a low temperature, in particular by the increase in the number of charging cycles.
In particular when the electric storage device is charged and discharged, a larger current flows therein as the internal resistance thereof becomes smaller. As a consequence, the deterioration of the electric storage device advances easier as a lower internal resistance of the electric storage device.
Therefore, the technique described in Patent Document 1 is undesirable from the standpoint of protecting the electric storage device. Accordingly, the feature of increasing the temperature by pumping a heating medium into the electric storage device 6 can be considered.
However, where the temperature of the electric storage device 6 is raised by using a heating medium, the circuit configuration for maintaining the engine, electric storage device, and equipment that needs cooling at the appropriate temperatures increases in size and complexity. The first reason therefor is that a warming-up circuit and a cooling circuit, for example, such as described in Patent Document 2, should be additionally provided as circuits that are made independent by respective pipes in order to adjust the temperature of the electric storage device 6 to the appropriate temperature. The second reason is that two heat exchangers, namely, for warming up and cooling, are needed. Therefore, such a system is very inconvenient from the standpoint of space and cost.
Patent Document 1: Japanese Unexamined Patent Publication No. 2010-127271
Patent Document 2: WO 2008/015798